Internal combustion engines come in a number of forms, the most common of which are spark ignited, gasoline fueled, engines and compression ignition ignited, diesel engines. Modern spark ignited, gasoline fueled, engines utilize air, either at atmospheric or above atmospheric pressure, and mix it with fuel, through an appropriate fuel metering system, to produce a mixture in the combustion chamber that is as close to stoichiometric as possible. The stoichiometric air-fuel ratio is the mass ratio at which all the fuel and all the air are combined in the combustion process. For gasoline fueled, spark ignited engines, the air-fuel ratio is 14.7-1. This characteristic enables appropriate exhaust aftertreatment using a three-way catalyst to reduce oxides of nitrogen in the engine exhaust.
The compression ignition, or diesel type, is used in many commercial and industrial engine power applications because of outstanding durability and fuel economy superior to the spark ignited, gasoline fueled, engine. The diesel engine utilizes the heat of compression of intake air into which a metered quantity of fuel is injected to produce combustion. The nature of the diesel engine cycle is that it has a variable air-fuel ratio that can under part power conditions rise to levels significantly above stoichiometric. This results in highly enhanced part power fuel economy since only the quantity of fuel needed for particular power levels is supplied to the engine.
While the diesel engine provides superior part-power fuel economy it has a combustion process making it more difficult to reduce the oxides of nitrogen with, for example, three-way catalyst. A conventional method to produce diesel engines having a stoichiometric air-fuel ratio would be to use a throttle normally used in a spark ignited gasoline engine to reduce intake air flow under part power conditions. The introduction of this throttling device, however simple, produces thermodynamic losses in the sense that the air into the engine is throttled and energy is wasted.
Accordingly, a need exists in the art to provide an internal combustion engine that system that minimizes losses that occur under part power conditions.